The following background information is provided to assist the reader to understand the environment in which the invention will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.
A spark plug is a device, inserted into the combustion chamber of an engine, containing a side electrode and an insulated center electrode spaced to provide a gap for firing an electrical spark to ignite air-fuel mixtures. The high-voltage burst from the coil via the distributor is received at the spark plug's terminal and conducted down a center electrode protected by an insulator. At the bottom of the plug, which projects into the cylinder, the voltage must be powerful enough to jump a gap between the center and side electrodes through a thick atmosphere of fuel mixture. When the spark bridges the gap, it ignites the fuel in the cylinder.
An alternative to spark ignition known in the art is torch jet-assisted spark ignition which, as taught by U.S. Pat. No. 4,924,829 to Cheng et al., U.S. Pat. No. 5,405,280 to Polikarpus et al. and U.S. Pat. No. 5,421,300 to Durling et al., offers several advantages over spark ignition approaches. Torch jet-assisted spark ignition employs a jet of burning gases that is propelled into the combustion chamber to increase the burning rate within the combustion chamber by providing increased turbulence as well as presenting a larger flame front area. As a result of a faster burning rate, lower cyclic variation in cylinder pressure is achieved, which enables a higher engine efficiency with a higher compression ratio.
In a torch jet-assisted spark ignition system, the jet typically emanates from a combustion prechamber, and passes through an orifice into the main combustion chamber. Though an air/fuel mixture can be introduced directly into the prechamber through a separate intake valve or fuel injector, it is generally preferable that the air/fuel mixture originate from the main chamber in order to simplify the construction of the engine and its ignition system. Furthermore, combustion of the air/fuel mixture within the prechamber can be initiated from within by a separate igniter, or can be initiated by the flame from within the main chamber. With either approach, combustion typically proceeds relatively simultaneously in both the prechamber and the main chamber. Because of the small relative volume of the prechamber, however, a high pressure is developed in the prechamber while the pressure is still relatively low in the main chamber. As a result, a jet of burning gases shoots from the prechamber far into the main chamber, and thereby significantly increases the combustion rate in the main chamber.
Currently used torch jet spark plugs, as taught by Durling et al., comprise a combustion prechamber on whose surface an inner electrode is formed. This inner electrode is formed by depositing a metal paste, such as a platinum or palladium metal paste, on the internal surface of the prechamber while the insulator body is in a green state prior to firing. During firing, the carrier component of the metal paste is dissipated, and the metal component wets and adheres to the internal surface of the prechamber to form a metal layer having a thickness of preferably about 0.01 to about 0.6 millimeters.
Polikarpus et al. teach an integrated molding and inking process for forming the inner electrode for the torch jet spark plug. This inner electrode is formed by the application of a metal ink to an outer surface of an elongated mandrel such that the metal ink forms a coating on the mandrel. The mandrel is then inserted into a suitable mold and the mold is filled with a substantially dry ceramic powder such that the powder envelopes the coating on the mandrel. The dry ceramic powder is then compacted so as to densify the dry ceramic powder and thereby form a “green” ceramic blank.
An ideal spark plug will always spark between 20,000–25,000 volts. Lower sparking voltages means decreased burn rate, high fuel consumption, and higher emissions. Higher sparking voltages around 30,000 volts or higher, leads to breakdown of the electrical system, spark plug wires, etc. Higher sparking also causes higher radio noise and radio frequency interference. During testing, the plugs produced by the Durling et al. and the Polikarpus et al. techniques did not perform as desired, often failing to meet the minimum desired 100 hours of accelerated testing. Thus, the typical life span of these spark plugs is approximately 10,000–30,000 miles. One of the factors leading to this premature failure appears to be deposits from the engine such as calcium phosphate at the tip. Another factor appears to be the quick erosion of the electrode due to the abrasive forces caused by “explosions” in the chamber. Electrode wear can cause premature spark plug failure and the voltage necessary for “sparking” may reach unacceptable levels.
There is a need in the industry to increase the durability of the spark plug electrode and to minimize the amount of electrode wear due to abrasive forces. A reduction in electrode wear would consequently increase the life span of the spark plug to greater than the 10,000–30,000 mile life currently achieved.